1. Field of the Invention
The invention relates to a method and apparatus for making heat exchangers, and more particularly, relates to a method and apparatus for making heat exchangers having internally grooved and externally finned coils which form a coil slab.
2. Discussion of Related Art
So called "heat pipes" are becoming increasingly popular for passive heat transfer for heating, cooling and/or dehumidification purposes. The typical heat pipe includes a plurality of vertical or inclined copper tubes thermally and mechanically connected to one another by stacks of perforated aluminum fins which also serve to promote heat transfer between the tubes and the surrounding environment. These tubes are charged with a refrigerant and arranged such that the upper and lower portions of the tubes are in thermal contact with relatively cool and relatively warm environments, respectively. Refrigerant in the lower portion of each of the tubes receives heat from the relatively warm environment, vaporizes, and rises into the upper portion, where it transfers heat to the relatively cool environment, condenses, and flows back into the lower portion, thus repeating the cycle. The lower portion of a heat pipe thus acts as an evaporator which cools and dehumidifies the surrounding environment, and the upper portion acts as a condenser which warms the surrounding environment. The efficiency of the tubes forming the coils of such heat pipes can be significantly enhanced by forming the tubes with internal grooves which, (1) increase the effective surface area of the inner surfaces of the tubes and thus increase heat transfer area, and (2) act as wicks increasing capillary action and promoting refrigerant flow through the tubes.
Heat pipes and other heat exchanger coils are normally constructed in the following manner. The tubes are inserted through a stack of perforated fins such that collars of the fins surround the tubes with narrow gaps formed therebetween. An expanding tool such as a hard metal ball is then forced through the tubes to expand them beyond their elastic limits into contact with the collars of the fins, thereby permanently increasing the tube diameters and forming a secure mechanical bond between the tubes and the fins. Grooves are scribed or otherwise formed in the tubes either prior to or following this expansion operation, and the tubes are charged with a refrigerant and sealed and/or connected to one another to form coils. The resulting assembly, often called a coil slab, is now ready for use as a heat pipe or another heat exchanger coil.
The primary disadvantage of the process described above is that the tubes must be expanded and grooved in separate steps using separate expanding and groove forming tools. This not only increases production time by at least a factor of two as compared to either the expansion or the groove forming operation, but also greatly increases tooling costs and labor expense.